An Australian teenager is in a critical condition in hospital following a New Year's Eve assault that took place in Sydney's King's Cross just metres from where another man was king-hit and killed.

Police will allege that around 9pm on Tuesday (11pm NZT), a 25-year-old man hit the 18-year-old with a single punch in an unprovoked attack, causing the younger man to strike his head on the pavement.

The teenager immediately fell unconscious and was not breathing when paramedics were called to the scene.

The incident occurred in front of the Mercure Hotel on Victoria Street, Potts Point around the same area where Thomas Kelly, also aged 18 at the time, was assaulted and later died from his injuries in 2012.

"When these things happen they are always a tragedy. I'm not about to compare this incident to Thomas Kelly," NSW Police Assistant Commissioner Mark Murdoch said.

"When anyone's assaulted it's a travesty. It is nothing but disappointing for police but also devastating for the young man's family.

"Even with police on every corner, we still cannot stop this kind of fight, particularly when people are affected by alcohol and particularly when they are intent on, we will allege, just going out and hurting people, as we will allege this [person] was."

A NSW Ambulance spokesman said that paramedics took the 18-year-old man to St Vincent's Hospital with critical injuries about 9pm on Tuesday.

He then underwent surgery and was returned to intensive care, a hospital spokesman said.

The young man's family is with him at the hospital where he remains in a critical condition.

Police arrested the 25-year-old man almost immediately following the assault and took him to Kings Cross police station.

He later was charged with maliciously inflicting grievous bodily harm, assault occasioning actual bodily harm and other assault-related offences. He is due to appear at Parramatta Bail Court on Wednesday.